Long haul fiber transmission systems have historically been limited by such factors as fiber loss, fiber dispersion, fiber nonlinearities, and amplifier noise. With the advent of practical optical amplifiers, these systems are effectively no longer subject to loss limitations. Rather, the significant system limitation has become dispersion. Present fiber optic transmission systems use transmitters employing direct current modulation of laser diodes. When modulated, these lasers produce pulses exhibiting large, uncontrolled wavelength shifts called "chirp." In the presence of a dispersive medium such as an optical fiber, the chirped pulses can be severely distorted when they finally reach a remote receiver.
System limitations imposed by dispersion may be countered by the use of dispersion-shifted fiber, "zero-chirp" transmitters, dispersion equalization, and soliton propagation. The use of dispersion-shifted fiber attempts to minimize dispersive loss within the transmission band whereas the use of zero-chirp transmitters attempts to maintain the transmission wavelength constant, preferably at the attenuation loss minimum for the fiber. Dispersion equalization, both at the receiver and at the transmitter, has been utilized to substantially compensate for the effects of dispersion on the transmitted pulses. Soliton propagation is dependent upon the presence of small amounts of dispersion at the transmission wavelength.
In the prior art, zero-chirp transmitters have been developed to ameliorate the problem caused by chirped pulses transmitted at high bit-rates over dispersive optical fiber. Generally, the term chirped pulses refers to pulses those whose wavelength swings dynamically about a central wavelength. The amount of chirp varies randomly as the transmitter operates thereby causing the chirped pulses to incur penalties in such systems. These penalties involve additional loss which limits the maximum transmission length or maximum transmission bandwidth due to intersymbol interference in the transmitted data pulses. In conventional optical fiber, the wavelength at which the fiber loss minimum occurs does not necessarily coincide with the wavelength at which zero dispersion occurs. Thus, when the lightwave transmitter produces optical pulses at a particular wavelength corresponding to the loss minimum of the fiber, wavelength chirping of the pulses causes pulse spreading because different wavelength components of the pulses experience different amounts of dispersive delay. Dispersion is higher for wavelength components of the pulses not at the zero dispersion wavelength. By removing chirp from the transmitted pulses, it has been thought possible to produce pulses with the minimum spectral width substantially at a single desired wavelength using a zero-chirp transmitter and thereby assure that only a small amount of pulse spreading is experienced. In general, such zero-chirp transmitters include an external modulator coupled to a laser. One external modulator which has been proposed is a lithium niobate, Mach-Zehnder interferometer using push-pull drive signals on the separate arms of the interferometer. See, for example, Koyama et al., J. Lightwave Technol., Vol. 6, No. 1, pp. 87 et seq. (1988) and Namiki et al., Proc. of Seventh International Conf. on Integrated Optics and Optical Fiber Communication, paper 19D4-2 (1989).
While the zero-chirp transmitter offers a potentially attractive solution for high bit-rate transmission at a wavelength "in the" optical fiber exhibiting non-zero dispersion, better performance has been predicted by using pulse compression techniques in the transmitter to achieve a dynamically varying chirp in a broad negative region. See Koyama et al., J. Lightwave Technol., Vol. 6, No. 1, FIG. 8, p. 91 (1988). Subsequent to this prediction, however, the art continues to express the need for lowering the transmitter chirp to zero as the method for combatting dispersion when communicating away from the zero dispersion wavelength of the optical transmission fiber. See, Okiyama et al., J. Lightwave Technol., Vol. 6, No. 11, p. 1686, 1691 (1988).